Lymphocytes isolated from hypothyroid patients contain high-affinity, limited-capacity, nuclear binding sites for triiodothyronine (T3). The estimated equilibrium dissociation constant was 3.08 × 10−11M. This value is virtually identical to that determined with GH1 cells, a rat pituitary tumor cell line, in culture. The maximal binding capacity was estimated to be 4 × 10−15 moles of T3 bound per 100 ug of lymphocyte DNA. This is equivalent to approximately 300 molecules of T3 bound per cell nucleus.